Full gigabit speeds?

I just got CenturyLink Gigabit service, and the fastest speeds I can get ising this adapter is around 650Mbps down/800Mbs up. The CenturyLink installer says it's a limitation of my hardware, and that only fairly new machines will actually get the full 1000Mbs / 1000Mbs speeds.

Mid-2012 MBP Retina, which has no ethernet so I'm using this adapter.

Does anyone know if there are limits in the adapter, older Macs, or...? If I get a new MBP but it turns out it's the adapter that is chocking things, then I won't see any benefit on that front.